The invention relates to a proximity switch immune to interference fields, with an oscillator circuit that can be influenced by a permanent magnet, which oscillator circuit is damped in its basic condition by a coil core consisting of an amorphous metal and which can be undamped by the approach of the permanent magnet, and with a first signal evaluating circuit in the form of a multivibrator stage, connected to the oscillator output, this signal evaluating circuit yielding an output signal corresponding to the undamped condition of the oscillator.
Such proximity switches immune to interference fields are required in cases wherein interference fields occur within the range of the proximity switch, which can lead to faulty switching operations. This is the case, for example, when using proximity switches for signaling the positions of pistons in pneumatic cylinders utilized in welding plants, specifically in remotely operated welding equipment. The power feed cables for the electrical welding units, extended past the pneumatic cylinders in the proximity of the switches, can generate such high magnetic fields that they result in faulty switching in unprotected proximity switches.
A proximity switch immune to interference fields has been known from EP-OS No. 01 79 384 wherein each proximity switch contains respectively two sensor portions, responsive to the approach, and wherein the difference of the signals in the two sensor portions is evaluated. On account of the spatial arrangement of the two sensor portions, the difference in the signals when generated by interference fields is substantially smaller than in case of signals produced by the approache. Therefor, the effects of interference fields are substantially diminished. This conventional arrangement, however, is possible only in case of proximity switches, the sensors of which cannot be magnetically saturated, or can be saturated only with difficulty. When using saturable sensors, the sensors in that arrangement would, in case of saturation, exhibit signals of the same level so that difference evaluation would no longer be possible.
Furthermore, DOS No. 2,815,711 discloses a proximity switch comprising an oscillator arrangement responsive to a metallic control lug. In order to prevent falsification of the output signal of the proximity switch unintentionally due to electromagnetic alternating fields, changes in the condition of the oscillator are purposely delayed with respect to time when transposed into the corresponding changes in the output signal of the proximity switch. In this conventional arrangement, suppression of faulty output signal conditions that could be caused by electromagnetic alternating fields is attained only if it can be assumed that the duration of time during which the interfering electromagnetic alternating fields are effective is limited and known. This is so in specific usages, for example in case it is certain that the interfering magnetic alternating fields are at a sufficiently low level for an adequate period of time in the proximity of their respective zero-axis crossings. The modulation limit of the amorphous metallic core is 1/100 to 1/1000 of that of a ferrite core. In sinusoidal modulation with magnetic fields, the metallic core is saturated within 1/100 to 1/1000 of the time during which a ferrite core is saturated. In case of a proximity switch with triggering by a permanent magnet, the time periods available for the timed setback of the activating delay times are so short that the timed gating in the zero-axis crossing of the interfering magnetic alternating field provided in this arrangement is impossible with proximity switches with triggering by permanent magnets.